1. Field of the Invention
The present invention relates to a method for manufacturing double-sided printed circuit boards including an insulating substrate having conductor patterns formed on both sides thereof.
2. Description of the Background Art
Printed circuit boards are used in a variety of electrical or electronic devices. Among the printed circuit boards are single-sided printed circuit boards including an insulating substrate of e.g. an insulating plastic having a conductor pattern made of a metal foil such as copper on one side thereof and double-sided printed boards including an insulating substrate having conductor patterns on both sides thereof.
In a double-sided printed circuit board, a via hole that is metal plated or the like is used to establish an electrical connection between the conductor patterns on the sides of an insulating substrate. Among via holes are through holes that are formed to pass through the conductor patterns on both sides and an insulating substrate, and blind via holes that are formed to pass through the conductor pattern on one side only and an insulating substrate.
JP 2003-8204 A discloses a method for manufacturing printed circuit boards that provide formation of fine patterns as well as improved flexural properties and dimensional accuracy, while preventing electrical connection between terminals. The method as disclosed in JP 2003-8204 is now described.
An insulating substrate with metal foils on both sides thereof is first prepared. Next, a through hole is formed so that it passes through the insulating substrate and metal foils on both sides, followed by the formation of an electroless copper plating layer on the surfaces of the metal foils.
Then, the metal foils are patterned by etching to form conductor patterns that include a peripheral region of the through hole. A plating resist is subsequently formed over the electroless copper plating layer except the portions on the inner surface and the peripheral region of the through hole, and then with the plating resist as a mask, an electrolytic copper plating layer is formed on the electroless copper plating layer on the inner surface and peripheral region of the through hole. After this, the electroless copper plating layer is removed except the portions under the electrolytic copper plating layer. Finally, the above is coated with a cover layer.
The double-sided printed circuit board thus formed according to the method described in JP 2003-8204 A exhibits high flexural properties, since the electrolytic copper plating layer is formed only on the inner surface and peripheral region of the through hole and not on the other regions.
The double-sided printed circuit board according to JP 2003-8204 A, however, shows a tendency for uneven thickness of the electrolytic metal plating layer on the peripheral region of the through hole.
This may result in deterioration of the reliability of electrical connection in connecting the double-sided printed circuit board with electronic parts such as semiconductor devices or other wiring circuit boards.